1. Field of the Invention
The present invention relates to the development of underground reservoirs such as petroleum reservoirs, gas storage sites or underground waters. In particular, the invention allows construction of reservoir models used for simulating fluid flows (hydrocarbons, water, CO2, etc.) in underground reservoirs and to eventually allow production or injection prediction, and optimization of the management of such underground reservoirs.
2. Description of the Prior Art
Optimization and development of petroleum reservoirs, for example, are based on the most accurate possible description of the structure, the petrophysical properties, the fluid properties, etc., of the reservoir. A tool known as a “flow model” allows accounting for these aspects in an approximate way. Such a model is a model of the subsoil, representative of both its structure and its behavior. Generally, this type of model is represented in a computer and is then referred to as a “numerical model.” A flow model comprises a grid, generally three-dimensional, associated with one or more petrophysical properties (porosity, permeability, saturation, lithology, etc.). The association assigns values of the petrophysical properties to each cell of the grid.
These models, which are well known and widely used in the petroleum industry, allow determination of many technical parameters relative to the study or the development of a reservoir, such as a hydrocarbon reservoir. In fact, since the flow model is representative of the structure of the reservoir and of the behavior thereof, engineers use it, for example, to determine which zones are the most likely to contain hydrocarbons, the zones in which it can be interesting/necessary to drill an injection or a production well in order to enhance hydrocarbon recovery, and the type of tools to use, the properties of the fluids used and recovered, etc. The interpretations of flow models in terms of “technical development parameters” are well known. Similarly, modelling CO2 storage sites allows monitoring of these sites, to detect abnormal behaviors and to predict the displacement of the injected CO2.
In general terms, the flow model is constructed on the basis of a fine geological description of the sedimentary layers that make up the reservoir. This geological description provides data to each cell in terms of lithologies. Lithologies are the various types of rocks present within the reservoir, such as sandstones, argillaceous sandstones and dolomites. There can be lithologic and sedimentary facies.
Once the model is described in terms of lithologies, the petrophysical properties required for flow simulations are then assigned according to these lithologies. A specific lithology will specifically have a particular porosity and permeability distribution, as well as saturations, relative permeabilities, capillary pressures, etc. These petrophysical properties generally result from laboratory analyses on rock samples taken from the reservoir (cuttings, cores).
Another type of petrophysical property necessary for flow simulations relates to the capillary pressure curves and the relative permeability curves. In this case also, assigning these physical properties to the cells of the flow model is done in close connection with the lithologic and sedimentary facies encountered on the corresponding cells. This assigning is also achieved from experimental measurements performed in the laboratory on rock samples taken from the reservoir. The experimental measurements lead in particular to a set of data relative to the capillary pressure and relative permeability curves.
In order to associate such curves with specific lithologies, it is common practice to sort them out manually, by observing them visually, so as to study the possibility of defining typical curves. That is, model curves representing, for example, the typical behavior of a given lithology. When this analysis is complete, when it is possible to classify the experimental curves measured in the laboratory and to define typical curves for the various lithologies present in the reservoir, the lithologies are said to be associated with rock types characterized by a very specific petrophysical behavior, in particular in terms of saturation, capillary pressure and relative permeability.
However, this visual comparison allowing the capillary pressure and/or relative permeability curves to be classified rapidly becomes difficult to achieve when there are a large quantity of experimental measurements. If several dozen experimental curves are available, or even several hundred, manual classification is difficult to implement and leads to very subjective results (related to the person leading the analysis) that are not really based on quantitative criteria.